This invention relates to a new and improved method and apparatus for controlling the operation of induction motor drive systems and in particular variable speed, current fed induction motor drive systems.
In current fed induction motor drive systems, the magnitude and the frequency of the alternating current that excites the stator windings of the motor are controlled, in contrast to systems of the voltage type wherein the magnitude and frequency of the applied alternating voltage are controlled. The controlled current is fed to the induction motor from the output terminals of suitable electric power conversion apparatus which is energized in turn by an available source of electric power, and the operation of the conversion apparatus is controlled by an associated control system so as to establish desired levels of current magnitude and frequency. Typically, the conversion apparatus comprises a phase controlled rectifier circuit whose input terminals are adapted to be coupled to a source of a-c power, an inverter whose output terminals are coupled to the stator windings of the induction motor, and a d-c link, including a current smoothing inductor or choke, connected between the respective d-c terminals of the rectifier circuit of the inverter. In such a system the magnitude of alternating current supplied to the motor can be controlled by retarding or advancing the "firing angle" of the controllable electric valves in the phase controlled rectifier circuit, and the frequency of this current can be controlled by appropriately varying the switching frequency of the controllable electric valves in the inverter.
By suitably controlling the magnitude and frequency of the excitation current in relation to the speed of the motor, the motor can be operated in a variety of modes, including constant torque and constant horsepower. For many applications it is useful to maintain the motor torque constant for motor speeds from zero up to a predetermined base speed at which the horsepower reaches a limit determined by the maximum power capability of the components in the inverter and rectifier or by the maximum power available from the source. If speeds above base speed are desired, the mode of operation must be changed to a constant horsepower mode wherein torque is reduced hyperbolically with increasing speed so as to keep the horsepower of the motor from exceeding its predetermined limit.
The torque of an induction motor depends on the amount of magnetic flux in the air gap between its stator and rotor and on the slip frequency therebetween. The effective slip frequency by definition is the difference between the frequency of the rotating flux wave in the air gap of the motor and the equivalent electrical frequency at which the motor shaft is rotating (i.e., motor speed). In steady state the frequency of air-gap flux rotation is the same as the stator excitation frequency (i.e., the frequency of the alternating current excitation which is supplied to the motor). The magnitude of air gap flux is generally proportional to the magnitude-to-frequency ratio of the stator voltage. At high torques in a controlled current induction motor control system, when motor current is no more than 30 to 45 electrical degrees out-of-phase with respect to voltage, there is a substantially direct relationship between the magnitude of alternating voltage at the stator terminals of the motor and the magnitude of direct voltage at the rectifier end of the aforesaid d-c link (i.e., at the d-c terminals of the rectifier circuit).
A constant torque operating mode can be advantageously obtained by maintaining the motor flux at a predetermined substantially constant level and by controlling the slip frequency in accordance with the desired torque as established by a variable torque command signal. As disclosed in the prior art (e.g., U.S. Pat. No. 3,863,121), constant flux is maintained by regulating the magnitude of stator current as a predetermined non-linear function of the torque command signal, which function is selected so that stator current will have the proper relation to slip frequency to maintain the aforesaid constant level of air gap flux in the motor regardless of its speed. With a relatively high slip frequency set by a correspondingly high torque command signal, and with constant flux, the stator voltage of the motor tends to increase with increasing speeds, thus necessitating a proportionate increase in the d-c voltage of the phase controlled rectifier circuit. Eventually a speed can be reached at which the firing angle of the rectifier valves is fully advanced and the d-c voltage is maximum, whereupon the current magnitude regulating loop becomes saturated. Since the current magnitude regulating loop is the stabilizing influence in the prior art control scheme, the system becomes unstable when the saturation point is reached.
One possible solution to the instability problem is to limit the maximum stator voltage to a level appreciably lower than the maximum voltage capability of the phase controlled rectifier circuit. A scheme for doing this has been disclosed in U.S. Pat. No. 3,769,564 wherein the slip frequency of the motor is increased proportionately with speed if the alternating voltage on the motor stator tends to exceed a predetermined limit. In this way, motor flux can vary inversely with speed above the base speed at which the voltage limit is reached, and consequently a relatively constant horsepower mode of operation is obtained. The difficulty with this solution to the stability problem is that it prevents the full power capabilities of the power source and of the phase controlled rectifier circuit from being realized, and it is subject to misoperation in the event of short-term reductions in source voltage. Additionally, the known controlled current inverter motor drive systems heretofore used have employed a separate phase controlled rectifier for each controlled current inverter/motor set, thereby, increasing the cost of the system greatly. To overcome these difficulties, the present invention was devised.